It is known that vehicle emissions are a major contributor to air pollution. In order to identify vehicles that are releasing excessive polluting emissions, many countries mandate annual vehicle emission inspections. To this purpose various vehicle emission inspection systems have been developed. Generally, these systems can be very expensive, and their operation can require a vast amount of labor and skill. Additionally, emission inspection systems have traditionally been operated in testing stations where the emissions are measured when the test vehicle is idling or running under artificially loaded conditions. Although such measurements provide general baseline information regarding a vehicle's emissions and state of repair, it is not necessarily representative of “real world” driving conditions.
Recently, remote emission sensing systems have been developed for detecting emissions of vehicles as they are driving on the road. For example, U.S. Pat. Nos. 5,319,199 and 5,498,872 to Stedman et al. discloses a remote sensing system in which the light source 1110 and detector 1130 are oppositely located on both sides of the road 1101, respectively, as shown in FIG. 6(a). For such an arrangement, a beam of light 1115 generated from the source 1110 passes through an exhaust plume 1140 emitted from a vehicle 1105 driven on the road 1101, thereby carrying an absorption signal associated with components and concentrations of the exhaust plume 1140. The beam 1115 is collected by the detector 1130 for analyzing the components and concentrations of exhaust plume 1140. Alternatively, as shown in FIG. 6(b), the light source 1110 and detector 1130 are located on the same side of the road 1101. And two reflectors 1150 located on the opposite side of the road 1101 are used to reflect the beam 1115 generated from the source 1110 to the detector 1130 with two passes through the vehicle exhaust plume 1140, which increases the absorption signal. This system measures only part of the plume and has to ratio the CO2 measurements to all other pollutants to get relative values. It does not measure the amount left behind or absolute values.
However, for such remote emission sensing systems, the source, detector and reflectors are set up on both sides of the road, and much care needs to be taken during their installation and maintenance. Additionally, such a system is difficult to operate with more than one lane of traffic particularly when more than one vehicle passes through the detector simultaneously. In other words, if multiple vehicles are present at the sensing location, each vehicle's exhaust plume may contribute equally to the emission measurement. Thus, on a single lane road, such as entrance and exit ramps, the existing remote sensing systems are not able to detect more than one exhaust plume at a time.
Furthermore, with current remote sensing systems the precision of the measurement can also depend on the position of the beam of light going across the road since the location of the vehicle's one or more exhaust pipes can vary from vehicle to vehicle. The precision of the emissions measured will vary depending on whether the beam is at the height of the tail pipe, or lower or higher where the exhaust has time to dilute before detection. With such an arrangement is also possible to miss the exhaust plume altogether.
Ultimately, the main drawback to current remote emission sensing is that since it only measures a portion of an exhaust plume it can only determine a plume's constituent gases and their relative concentrations. While such results can indicate if a vehicle is in need of repair, existing systems are not able to measure absolute amounts of emission components. Measuring absolute amounts of components is important since a surfeit can lead to severe air pollution. It is for this reason that many countries statutorily limit the amount of gas pollutants allowed in emissions. In fact, state and federal vehicle emissions standards and control requirements are stated in “grams per mile.” With existing systems this value must be extrapolated from the ratios reported by identifying the vehicle make and model and making assumptions about whether the vehicle is running rich or lean, the load on the vehicle, etc.
Quantitative imaging of gas emissions techniques has been patented. For example, U.S. Pat. No. 5,319,199 describes an elaborate system which uses gas self-emission radiation and gas filled cells. Unfortunately, the complexity of this method is unnecessary and cost prohibitive.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.